The present invention relates generally to multiple element optical amplifier arrays used to achieve a high output power laser beam and in particular, to a system to enable coherent beam combination of laser amplifier arrays.
The intensity and, hence, the power available from a single-mode optical fiber is limited by either optical surface damage or nonlinear optical effects. The limitations to achieving a high power fiber laser system can be overcome by coherent beam combination of the output power from multiple, broader linewidth optical fiber amplifiers. In a master oscillator power amplifier (MOPA) configuration having multiple fiber amplifier legs, the optical path lengths of each of the amplifier legs have to be controlled to a tighter and tighter tolerance level as the linewidth of the source increases. In addition, fine control of the optical phase is required in order to enable coherent combination of the individual amplifier outputs into a single, high-power beam. As a result of time varying thermal loads and other disturbances, an active feedback system is required in order to provide for both coherent addition and rapid slewing of the final beam direction.
Two systems for electronic phase locking of optical arrays to achieve a high power beam is the subject of three patents by one of the present inventors (T. M. Shay, “Self-referenced Locking of Optical Coherence by Single-detector Electronic-frequency Tagging,” U.S. Pat. No. 7,187,492 B1, T. M. Shay, “Self-Synchronous Locking of Optical Coherence by Single-detector Electronic-frequency Tagging,” U.S. Pat. No. 7,058,098 B1, and T. M. Shay, “Self-referenced Locking of Optical Coherence by Single-detector Electronic-frequency Tagging,” U.S. Pat. No. 7,233,433 B1). These patents are hereby incorporated by reference.
The Self-Synchronous Locking of Optical Coherence by Single-detector Electronic-frequency Tagging (Self-Synchronous LOCSET) system (U.S. Pat. No. 7,058,098 B1) provides a simple and robust method that needs only a single detector and has no reference leg. The phase errors between the individual array elements self adjust so that the mean phase error is zero, thereby maximizing the power in the central lobe. The Self-Referenced LOCSET system is also a simple and robust method that needs only a single detector, but has one reference leg. The phase-modulated (slave) array elements are adjusted to track the phase of the unmodulated (reference) element. Both embodiments of LOCSET can easily be scaled to large numbers of array elements because the scaling is done in the electronic instead of the spatial domain. This technology is applicable to general systems of laser amplifiers, i.e., semiconductor, bulk solid state, gas, dye, as well as fiber amplifiers.
Self-Synchronous LOCSET
In the Self-Synchronous LOCSET system, the phases of the individual array elements self-adjust so that the mean array phase error is zero. A block diagram of the Self-Synchronous LOCSET is shown in FIG. 1. The diagram begins with a master oscillator 1. There may or may not be an optical amplifier incorporated in the master oscillator laser. The output power from the master oscillator is divided by a 1×N power splitter 2. Each of the N output signals from the 1×N power splitter 2 are then directed to N optical modulators 3 where each of the N signals is phase modulated by a unique electronic frequency. The modulation frequencies must be selected so that beat notes of the N elements can be uniquely isolated. The errors signals for each of the N elements are fed to N phase adjusters 4 and then to optical amplifiers 5. The optical modulators 3 and the optical phase adjusting elements 4 can be separate devices, or they may be single devices that perform both tasks. The outputs from the optical amplifiers 5 are then directed to the array output optics 6. The relative position of the optical amplifiers 5, the optical modulators 3, and the phase adjusters 4 are in principle arbitrary. However, practical details such as power handling capability of individual elements, system noise characteristics, or even reduction of the modulation effectiveness by succeeding elements may lead to a preferred sequence of optical elements.
Next, the N amplified outputs are optically co-aligned in the alignment optics 6 and the output is directed to the beam sampler 7. In the case where the output array optics 6 contains a beam combiner, the output may be a single beam. In a variation of this embodiment, the beams may be sampled before the alignment optics. In that case the beam sampler 7 may precede the co-alignment optics 6. While the majority of the power passes through the beam splitter 7 and constitutes the array output 8, a small fraction of the array output is directed to the far field imaging optics 9 and then on to the photodetector 10. The imaging system 9 is used to image a far-field central spot onto the photodetector 10 that fills or overfills the active area of the photodetector 10. The signal-to-noise ratio for a given optical power in the sampled beam is optimized when only the central lobe of the far field is imaged onto the photodetector. However, this is not a necessary condition for achieving accurate phase control. Excellent phase locking can be achieved when the central lobe overfills or underfills the photodetector active area. It is only necessary that there be a sufficient signal-to-noise ratio for phase locking.
The electrical signals from the photodetector 10 are signal processed 11 to isolate and extract the optical phase control signals for each array element, and the optical phase control signals are then applied to the phase adjusting elements 4 using negative feedback to cancel the phase difference between the array elements. Optimum performance is obtained when the array elements are all traveling in the same direction with the same divergence. However, the optimum condition is not required for excellent phase control to be demonstrated.
The output power of a narrow linewidth fiber amplifier is limited by Stimulated Brillouin Scattering (SBS). The SBS threshold is currently the limiting nonlinear process in single frequency fiber amplifiers. A simple way to mitigate Stimulated Brillouin Scattering and increase the power available from a single fiber amplifier is to broaden the master oscillator linewidth beyond the SBS line width. While this is an effective technique for mitigating SBS it presents problems for active coherent beam combination since the coherence length is inversely proportional to the linewidth and decreases as the linewidth increases.
The output power of an amplifier system using Self-Synchronous LOCSET for phase locking may, therefore, be enhanced if the master oscillator linewidth is broadened beyond the Stimulated Brillouin Scattering (SBS) linewidth. This allows the output powers of the individual fiber amplifier legs to be increased with a consequent, increase in the total output power of the fiber laser system. However, the optical path length of each amplifier chain must then be matched to within the coherence length of the master oscillator. The enhancement of LOCSET to enable simultaneous optical phase and path length matching is a subject of the present invention. The invention may also be used to path length match and phase lock passive optical systems and to measure path length differences in broad line interferometric applications, such as optical coherence tomography.
Self-Referenced LOCSET
In the Self-referenced LOCSET system (U.S. Pat. No. 7,187,492 B1 and U.S. Pat. No. 7,233,433 B1), one signal is designated a reference signal while the M=N−1 remaining (slave) signals are optically phase adjusted relative to the reference. A diagram of this system is shown in FIG. 2. The diagram begins with a master oscillator 21. There may or may not be an optical amplifier incorporated in the master oscillator laser. The output power from the master oscillator is divided by a 1×N power splitter 22. The M output slave signals from the 1×N power splitter 22 are then directed to M optical modulators 23 where each of the M signals is modulated by a unique electronic frequency. The remaining reference signal from the 1×N power splitter 22 is also sent through an optional optical modulator 32 and is designated the reference signal. The modulation frequencies must be selected so that beat notes between the reference and the other M slave elements can be uniquely isolated. The M slave signals are fed to M phase adjusters 24 and then to optical amplifiers 25. The reference signal proceeds directly from the 1×N power splitter 22 through an optical modulator 32 which may not be present to an optical amplifier 25. The optical modulators 23 and the optical phase adjusting elements 24 can be separate devices, or they may be single devices that perform both tasks. The outputs from the optical amplifiers 25 are then directed to the array output optics 26. The relative positions of the optical amplifiers 25, the optical modulators 23, and the phase adjusters 24 on the slave legs are in principle arbitrary. The relative positions of the optical amplifier 25 and the optical modulator 32, if it exists, on the reference leg are also arbitrary. However, for both the slave and reference legs, practical details such as power handling capability of the individual elements, system noise characteristics, or even reduction of the modulation effectiveness by succeeding elements may lead to a preferred sequence of optical elements.
Next the N amplified outputs are optically co-aligned in the alignment optics 26 and the output is directed to the beam sampler 27. In the case where the output array optics 26 contains a beam combiner, the output may be a single beam. In a variation of this embodiment, the beams may be sampled before the alignment optics. In that case the beam sampler 27 may precede the co-alignment optics 26. While the majority of the power passes through the beam splitter 27 and constitutes the array output 28, a small fraction of the array output is directed to the far field imaging optics 29 and then on to the photodetector 30. The imaging system 29 is used to image a far-field central spot onto the photodetector 30 that fills or overfills the active area of the photodetector 30. The signal-to-noise ratio for a given optical power in the sampled beam is optimized when only the central lobe of the far field is imaged onto the photodetector. However, this is not a necessary condition for achieving accurate phase control. Excellent phase locking can be achieved when the central lobe overfills or underfills the photodetector active area. It is only necessary that there be a sufficient signal-to-noise ratio for phase locking.
The electrical signals from the photodetector 30 are signal processed 31 to isolate and extract the optical phase control error signals for each array (slave) element, and the optical phase control signals are then applied to the phase adjusting elements 24 using negative feedback to cancel the phase difference between the array (slave) elements and the reference element. Optimum performance is obtained when the array elements are all traveling in the same direction with the same divergence. However, the optimum condition is not required for excellent phase control to be demonstrated.
The output power of an amplifier system using the Self-Referenced LOCSET phase-locking system may also be enhanced if the master oscillator linewidth is broadened beyond the Stimulated Brillouin Scattering (SBS) linewidth. Because the output powers of the individual fiber amplifier legs are increased due to a higher threshold for Stimulated Brillouin Scattering, the total output power of the overall fiber amplifier system will be increased. Again, the optical path length of each amplifier chain must then be matched to within the coherence length of the master oscillator. This enhancement is also a subject of the present invention.
Recently, Goodno et al Optics Letters, Vol. 35, No. 10, pp. 1542-1544, 2010 demonstrated an actively phase-locked coherent beam combining system emitting 1.43 kW of single-mode power when seeded with a 25 GHz linewidth master oscillator. However, efficient beam combination required that the optical path lengths be matched to within 1-mm. To achieve this path length matching mechanical optical trombones were used. However, mechanical optical trombones are devices that are vibration sensitive and therefore require very rigid and expensive vibration isolation systems. Mechanical sensitivity may make these systems unsuitable for many potential applications. An alternative method for matching the optical fiber path lengths is to splice additional fiber onto the fiber system to match the optical paths. While this sounds straight forward it is difficult to do with current fiber fusing technology because the fiber length added can generally be controlled only to an accuracy of a few centimeters due to uncertainty in the quality of the fiber cleaving and fusing processes. Thus this approach is not suitable for path length matching of a large number of broad linewidth optical fibers since the optical paths must be matched to within less than the coherence length which may be quite small.
Another approach is to utilize the fiber coupled optical trombone described by Yao in U.S. Pat. No. 7,534,990 B2 to obtain path length matching. An additional technique for optical path length matching is to utilize a piezoelectric fiber stretcher to bring about small changes in the path length of the optical fiber. (See Yao et al, U.S. Pat. No. 5,723,856, Yao et al, U.S. Pat. No. 5,929,430, Ichenko et al, U.S. Pat. No. 7,187,870 B2, and Sayyah U.S. Pat. No. 7,324,256 B1.) However, these devices have two problems, first they are microphonic as well as vibration sensitive and second their dynamic range is generally limited to small changes in the path length.
As mentioned above, broader linewidth lasers can effectively mitigate SBS, thus eliminating the major effect that limits the output power of beam combinable fiber lasers. This, however, incurs the added penalty of requiring accurate path length matching systems. Because vibration sensitivity is an issue for some applications, some forms of path length matching will be unsuitable. Because the present invention is an all-fiber system that doesn't contain any mechanical devices or free space optics, excellent performance is expected in an environment where vibrations are present. Finally, it is generally assumed that unless all of the path lengths are matched, the control loop will not operate. While this is generally true, the present invention circumvents this by utilizing a technique that separates out the control loop signal for each array element, i.e., the incoherence induced by any array element beams that are not path length matched will not interfere with the phase locking of those that are path length matched.